Dinosaurs and Birds. The Dinosaur Encyclopedia. A good discussion of the history of the bird-dinosaur link, including the various hypotheses for the origin of birds, which dinosaur groups may be the descendants of birds, and even the idea that dinosaurs descended from birds (rather than the other way around).
Are Birds Really Dinosaurs? University of California, Berkeley. Short discussion of dinosaur-bird relationships, with a good discussion of the history of this hypothesis, a list of 20 shared characteristics, and also some objections to the hypothesis. (Note: These objections were raised before the current findings of feathered dinosaur fossils from China).
Evolutionary Transitionals--From Dinosaurs to Feathered Birds. Jeff Poling. Part of the Darwinia and Evolution site. Good summary of early fossil birds, and feathered dinosaurs arranged on a cladogram, and highlighted based on appearance of no feathers, early feathers, or modern feathers.
China's Feathered Dinosaurs. Yale-Peabody Museum. Good overview of Sinosauropteryx, with pictures of the fossil remains, as an example of a theropod dinosaur with primitive "downy-like" plumage. To see more development of feathers on dinosaurs, you can compare Sinosauropteryx to Caudipteryx, which had well-developed arm and tail plumage.
Down Feathers and/or Display Feathers on Theropod Dinosaurs? University of California, Davis. Scroll down this discussion of evidence for warmblooded dinosaurs to the subheading concerning feathered dinosaurs. Good collection of online news articles and summaries of the different feathered dinosaurs that have been found in China.
Feathers, Scutes, and the Origin of Feathers. Jeff Poling. Explanation of the theory that feathers evolved from reptile scute scales.
All Mixed Up. Science Online. Summarizes the faked feathered dinosaur from China, which was errantly published in National Geographic, and for which National Geographic later had to apologize. Numerous actual feathered dinosaurs have been found from China, but this fake perpetuated the perception that many of the bird-dinosaur link fossils are frauds. To see reactions to the announcement on creationists' Web sites, see Answers in Genesis's latest "feathered" dinosaur hoax, or the Institute for Creation Research's Archaeoraptor doesn't fly
Archaeopteryx , Ancient Birds, and Dinosaur-Bird Relationships. Journal of Dinosaur Paleontology. This page provides links to numerous articles and technical comments concerning the origin of birds and relationship between birds and dinosaurs.